1. Field of the Invention
The present invention relates to a substrate mounting structure and a substrate processing apparatus, and in particular to a substrate mounting structure having a mounting stage on which a substrate is heated.
2. Description of the Related Art
As substrate processing apparatuses that heat wafers as substrates, substrate processing apparatuses are known that have a processing chamber that houses a wafer and subjects the wafer to predetermined processing, and a susceptor as a mounting stage which is disposed inside the processing chamber and on which the wafer is mounted, the susceptor having therein a heater. In such a substrate processing apparatus, the susceptor is supported by a cylindrical supporting member (stem), and a power source is connected to the heater via a feed line (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2003-124201).
Examples of the predetermined processing carried out on a heated wafer include reduction processing and Low-k restoration processing. Hydrogen gas is used in the reduction processing, and silane-based gas is used in the Low-k restoration processing. The hydrogen gas and the silane-based gas are explosive gas that tends to explode at a certain predetermined temperature (explosion limit temperature) or higher. To avoid the explosion of the explosive gas, while a wafer is being subjected to the Low-K restoration processing and the like, the temperature of the wafer being heated has to be made lower than the explosion limit temperature of the explosive gas.
Accordingly, as shown in FIG. 3, a substrate processing apparatus 102 has been developed in which temperature sensors 101 are embedded in a susceptor 100, and software operating on a PC monitors indirectly the temperature of the wafer W by monitoring the temperature of the susceptor 100 measured by, for example, a thermocouple, and when the temperature of the wafer W is likely to exceed the explosion limit temperature, the software provides control to stop the supply of electrical power to a heating element. In this substrate processing apparatus 102, the two independent temperature sensors 101 are embedded in the susceptor 100 to realize two temperature measurement lines so as to improve the reliability of heating-stop control. Moreover, the temperature sensors 101 are arranged in the central part of the susceptor 100 corresponding to the central part of the wafer W, and more specifically, at the intersection of the susceptor 100 and a stem 103 so as to improve the reliability of correlation between the measured temperature of the susceptor 100 and the temperature of the wafer W.
In the above described substrate processing apparatus 102, however, when the software runs out of control, heating-stop control cannot be properly carried out. Moreover, the possibility that the software may run out of control cannot be reduced to zero.
To cope with this, in recent years, it has been contemplated that heating-stop control is carried out without using software so as to improve the reliability of heating-stop control. Specifically, it has been contemplated that a thermostat, a temperature fuse, or the like is embedded in a susceptor, and the thermostat or the like mechanically breaks a feed line connected to a heating element depending on the temperature of the susceptor when the temperature of the susceptor (wafer) is likely to exceed the explosion limit temperature.
However, the thermostat has to be away from an atmosphere in a processing chamber, for example, a processing gas, and hence in a substrate processing apparatus that carries out heating-stop control without using software, a part of the susceptor at which the thermostat is disposed has to be covered with a stem. On the other hand, because the thermostat is larger in size than a temperature sensor, the stem has to be made thicker than the stem of the conventional substrate processing apparatus so as to cover the above-mentioned part with the stem.
Making the stem thicker increases the heat capacity of the stem, and hence the stem absorbs a considerable amount of heat generated by the heating element of the susceptor, and further, the stem functions as a heat flow path that connects the susceptor to the processing chamber, so that the most part of heat generated by the heating element is transferred to the wall of the processing chamber. As a result, heat is not properly transferred to the wafer, and hence it is difficult to maintain the temperature uniformity of the wafer.